Interference-limited systems have proliferated in recent years. These systems give users the convenience of real-time communications without the physical constraints of dedicated lines. In these systems, the data rate that can be achieved is dependent upon the amount of interference with a “transmission” (e.g., a cellular telephone call). The amount of interference is reflected in the quality of the channel (“channel quality”). The channel quality for a given transmission depends upon several factors, including path loss between a base station (e.g., a cell) and a terminal unit (e.g., a cellular phone), and interference from other transmissions within the same network.
Channel quality metrics such as the carrier-to-interference ratio (the “C/I ratio”), the bit error probability, and the coefficient of variation of the bit error probability, are often used as indicators of the data rate that can be achieved in a given transmission. Higher data rates can be achieved when the channel quality improves, e.g., at higher C/I values.
While interference is a limiting factor, the bandwidth available to these systems is also limited. When there is more than one user competing for part of the available bandwidth, methods and devices must be devised which allocate bandwidth efficiently and appropriately. Typically, users will be competing for bandwidth when they seek to send or receive transmissions at substantially the same time. For example, before uplinking, competing users will typically request authorization to transmit. Similarly, before downlinking, a base station controls transmissions to be sent to competing users. Any method must prioritize both uplink and downlink transmissions. Taking various factors into consideration, uplinked and downlinked transmissions can be authorized in accordance with a prioritization schedule or the like.
Prior to proceeding, it should be understood that the word “scheduling” may mean prioritizing, authorizing, or some combination of the two.
Scheduling methods have been the subject of significant study. A number of methods have been proposed, including “first-in-first-out,” “weighted fair queuing,” “round-robin scheduling,” and “virtual clock.” Under existing methods, users transmitting over channels whose channel quality is low are not penalized, but are protected and given a fair share of the bandwidth. These “fairness methods” are designed to allocate additional transmission time to users transmitting via such channels in order to allow the transmission of approximately the same amount of data as users making use of channels whose quality is much higher.
In developing these fairness methods, however, it has been assumed that the link level characteristics of each user's channel are independent of the scheduling method used. This assumption, though, neglects to account for the fact that the scheduling method has a considerable impact on the channel quality (i.e., achievable data rate) achieved by each user. That is, when a user is transmitting, the transmission itself causes interference to other transmissions. As a result, the longer such a user is transmitting, the more interference other users experience. Because the amount of interference is dependent upon the scheduling method, other users' achievable data rates will be reduced, which in turn increases the amount of time they spend transmitting, ultimately causing even more interference to other users.
Accordingly, it is desirable to provide methods and devices for authorizing transmissions in interference-limited networks that take into account the interdependence between scheduling and interference.
Other desirable features will become apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings and claims.